Greetings,

Adding another Google Play account to your device allows you to back up your game with a new slot. 

For another Google Play account, just add another Google account altogether. To do so, please follow these steps:

Force-stop the game by entering your device's Settings > Apps > the Game, and press 'Force stop'.
Enter your device Settings.
Go to Accounts, and press 'Add account'.
Add a Google account, either a new one (that you can create right away) or an existing one (that you have created earlier), but if you add an existing account please make sure it has not already been used for saving a backup of the game.
Follow the on-screen instructions to add the account. If you are adding an